There are two types of flight control on board aircraft:
primary controls which serve to control the immediate movements of the airplane; these are generally moving surfaces situated at the trailing edges of the wings; PA1 secondary flight controls which serve to trim the aerodynamic configuration of the aircraft to match different stages of flight. This category includes the tail plane and high-lift moving surfaces and tips. PA1 "Actionneurs--Des materiaux piezoelectriques pour les commandes du futur" [Actuators--piezoelectric materials for future controls], published in Usine Nouvelle, Oct. 31, 1996, No. 2568; and PA1 the unit blocks work in elongation/retraction and they are stacked with polarization that alternates in the height direction of the tubular structure; PA1 the stacks are separated in pairs by separator means extending along the height direction of the structure, and the separator elements are constituted by a succession of slabs each being of a height that corresponds at least to the height of two unit blocks, the slabs being suitable for sliding relative to one another and being of stiffness that is greater than that of the unit blocks, the separator zones between superposed slabs at the same height being offset in the height direction of the structure from one separator means to the next in the circumferential direction; PA1 the separator means are constituted by strips presenting a plurality of slots that, in pairs, define the slabs; PA1 the separator means are constituted by a plurality of separator elements which are superposed, each of which constitutes a unit slab; PA1 the electrode-forming means are constituted by the separator means; PA1 the unit blocks operate in shear; PA1 two successive unit blocks in the height direction of a given stack present electrode-forming metallizations on their facing faces; PA1 the stacks of unit blocks are separated by electrically conductive separator means to which the electrode-forming metallizations are connected; PA1 the unit blocks are distributed in washer-like layers in the height direction of the tubular structure; PA1 the actuator includes a prestress envelope in which the tubular structure of unit blocks is placed; PA1 the prestress envelope comprises a plurality of rings each providing prestress to a washer-like layer of unit blocks of the tubular structure; PA1 the rings are electrically conductive and the actuator comprises a plurality of contact areas distributed in the height direction of the structure between the separator means and the prestress rings; PA1 the contact areas are distributed so as to make contact with every other slab; PA1 the rings are separated by electrically insulating washers; and PA1 the envelope or the prestress rings is/are made of an alloy having shape memory. PA1 a) insulating washers and prestress rings in the low temperature phase are stacked in alternation inside an outer tube; PA1 b) split metal strips are stuck along generator lines of a cylindrical inner core having an insulating surface, adjacent pairs of strips being offset relative to one another in the length direction of said core by half the thickness of the core; PA1 c) the core is inserted together with its strips into the tubular structure constituted by the prestress rings and the washers; PA1 d) the unit blocks are inserted into the housings between the rings, the strips, and the core; and PA1 e) the assembly is heated to change the phase of the prestress rings. PA1 the active structure is a tube of magnetostrictive material having a plurality of slots disposed in a staggered configuration and defining between them both zones corresponding to the blocks that are designed to be deformed by the magnetostrictive effect, and zones corresponding to the separator slabs; PA1 the actuator includes means for applying a uniform permanent radial magnetic field to the active structure and for adding to said permanent field a radial control field which alternates from one block that is to be deformed by the magnetostrictive effect to another; PA1 the actuator comprises means for applying a permanent radial field to the active structure that alternates from one block which is designed to be deformed by the magnetostrictive effect to the next, and in adding to said permanent field a radial control field that is uniform; PA1 the actuator has permanent magnets in register with different blocks designed to be deformed by the magnetostrictive effect; PA1 the permanent magnets are of polarization that alternates from open magnet to the next; PA1 the permanent magnets are carried by a core which comprises a plurality of segments suitable for turning relative to one another, and the actuator further includes means for communicating the twisting movement of the active structure to said segments; PA1 said means comprise an axial torsion bar; and PA1 said means comprise elastomer links between successive permanent magnets in the height direction of the active structure.
The characteristics required of actuators for such surfaces are very different.
The primary controls must be capable of accommodating a passband that is greater than the spectrum width of the movements that the aircraft can perform, they must be capable of operating at all times, and they must be suitable for returning to a neutral position if there is a loss of power.
In contrast, secondary controls operate intermittently so they need a small passband only, but they must be capable of remaining in the most recent position in the event of a power failure.
These controls are generally implemented as hydraulic devices which, for primary controls, act directly, while for secondary controls they act via mechanical stepdown means. It is the mechanical stepdown means used in the second case that provide the required non-reversibility.
For various reasons (maintenance, pollution, fire risk, etc.) aircraft manufacturers are seeking to reduce the hydraulic contribution in controls and to promote electric control. Unfortunately, the technology of electromagnetic motors associated with stepdown means leads to equipment of excessive mass.